Antibodies against DNA modified by benzo(a)pyrene diol-epoxide I (BP-DNA) were raised, purified and studied in several ways. These antibodies did not react with double- or single-stranded DNA or with BP-RNA or BP-protein. Such antibodies were used in initial attempts to detect repair of BP-DNA in treated cells by fluorescence assay, to detect BP residues on DNA by electron microscopy, to detect BP residues on the DNA of gently permeabilized cells. The feasibility of a monoclonal system for producing such antibodies is under investigation.